Apparatus to control color registration and image density

ABSTRACT

An apparatus to control color registration and image density and a method of calculating a color registration error. The apparatus includes registration marks formed on a transfer belt such that each of the registration marks includes a scan direction component and a slanting direction component at an angle with respect to both the scan direction and the cross-scan direction. The apparatus further includes image density marks formed on the transfer belt, having predetermined image densities, and a registration and image density sensor provided above the registration marks and the image density marks to radiate beams onto the registration marks and the image density marks. The sensor receives beams reflected from the registration marks and the image density marks to produce detection signals, and obtains registration information and image density information from the detection signals. Accordingly, X-offset, Y-offset, printing width error, and skew can be simultaneously compensated for using a single apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Korean Application No.2001-54151, filed Sep. 4, 2001, in the Korean Industrial PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an apparatus to control colorregistration and image density in a printer, and a method of calculatingcolor registration error, and more particularly, to a color registrationcontrol apparatus to accurately detect color registration to provide ahigh quality picture, a method of calculating color registration error,and an apparatus to simultaneously detect both color registration andimage density.

[0004] 2. Description of the Related Art

[0005] Image forming apparatuses such as printers and copy machines forma latent electrostatic image by charging a photoconductive member on atransfer belt and performing selective exposure by scanning a laserbeam, develop the latent electrostatic image using colored toners and adeveloper unit, and transfer the developed latent electrostatic image toa recording medium by pressing and heating, thereby forming an image.

[0006] Generally, the colors of toners used in a developer unit are cyan(C), magenta (M), yellow (Y), and black (K). The four color toners aretransferred such that the four colors overlap to form a complete image.To deliver high quality images, unit images of individual colors shouldbe accurately superimposed. This superimposition of colors is referredto as color registration.

[0007] Color registration errors can arise from complex causes such asmismatch of the individual color units of a developer unit, errors inprocessing an optical lens, and motion errors of a transfer belt.Particularly, color registration error becomes a problem in an imageforming apparatus having a serial structure including a plurality ofdeveloper units.

[0008] Color registration errors have four types: X-offset, Y-offset,printing width error, and skew. X-offset arises in a scan direction inwhich a sensor scans. Y-offset arises in a cross-scan direction in whicha transfer belt moves. Printing width errors arise from a difference inwidth of an image area. Skew arises from displacement of a developmentline. In order to obtain high quality images using color registration, asensor to detect color registration errors and a method of accuratelycalculating the errors are required.

[0009]FIG. 1 is a diagram of a color registration sensor and a markpattern disclosed in U.S. Pat. No. 5,287,162. Referring to FIG. 1, acolor registration mark pattern 13 in a chevron shape is formed on atransfer belt (not shown). A split sensor 11 including two split cells11 a and 11 b detects a beam reflected from the color registration markpattern 13. Reference numeral 32 indicates a cross scan direction.

[0010] The split sensor 11 includes the two split cells 11 a and 11 band is designed to compare the amount of light reflected from the colorregistration mark pattern 13 and detected by the cell 11 a, with theamount of light reflected from the color registration mark pattern 13and detected by the cell 11 b, to produce an output when the quantitiesare the same.

[0011] The split sensor 11 must be aligned parallel to the colorregistration mark pattern 13, and formed in a chevron shape to detectdifferent colored chevron marks. Accordingly, the split sensor 11 isexpensive. In addition, when the split sensor 11 is not parallel to thecolor registration mark pattern 13, beams reflected from the colorregistration mark pattern 13 cannot be accurately detected.

[0012]FIG. 2 is a diagram of a color registration sensor and a markpattern disclosed in U.S. Pat. No. 5,631,686. Referring to FIG. 2, ablack swath 17 is laid on an intermediate transfer belt 19. Marks 25corresponding to yellow, cyan, and magenta are formed on the black swath17 in a chevron shape. A black mark 23 can be made by forming a void onthe black swath 17. A bicell sensor 21 is provided above theintermediate transfer belt 19.

[0013] The bicell sensor 21 detects the marks corresponding to yellow,cyan, and magenta based on the difference between the low reflectivityof the black mark 23 and the high reflectivity of each of the marks ofdifferent colors, and detects the black mark 23 based on the differencebetween the low reflectivity of the black mark 23 and the highreflectivity of the intermediate transfer belt 19.

[0014] The bicell sensor 21 can easily detect the black mark 23 due tothe great difference between the reflectivity of the black mark 23 andthe reflectivity of the marks 25 corresponding to yellow, cyan, andmagenta, but has difficulty detecting the marks 25 corresponding toyellow, cyan, and magenta since the difference in reflectivity betweenthe marks 25 is small.

[0015]FIG. 3 is a diagram of a color registration sensor and a markpattern disclosed in U.S. Pat. No. 5,909,235. Referring to FIG. 3, colorregistration mark sets 33 and 35 are provided at one side of a transferbelt 29. A wide area beam (WAB) sensor 31 to detect beams reflected fromthe color registration mark sets 33 and 35 is provided on the transferbelt 29. Here, reference numerals 72 and 34 denote image areas.Reference characters X and Y denote a scan direction and a cross-scandirection, respectively.

[0016] For each of the color registration mark sets 33 and 35, aplurality of black marks are formed first, and yellow, cyan and magentamarks are arranged in line with each of the black marks.

[0017] The WAB sensor 31 radiates beams at the color registration marksets 33 and 35, measures the reflectivity of the color registration marksets 33 and 35, and compares the areas of reflected beams with eachother, thereby producing a detection signal. The WAB sensor 31 does notfocus beams on the color registration mark sets 33 and 35, but diffuselyradiates beams onto sets 33 and 35 to detect beams reflected therefrom.

[0018] Since the WAB sensor 31 diffusely radiates beams, all of thebeams regularly and irregularly reflected from the color registrationmark sets 33 and 35 are detected. Accordingly, detection errors havingnoise components may increase according to the surrounding conditions ofthe transfer belt 29. In addition, radiated beams have multiplewavelength bands, so the sensitivity of the WAB sensor 31 is not uniformfor different wavelengths. As a result, the light receiving sensitivityof the WAB sensor 31 is not uniform, which decreases the accuracy of adetection signal.

[0019] In addition to color registration, i.e., arrangement of colors injuxtaposition, it is also necessary to appropriately adjust imagedensity in order to obtain high quality images. Conventionally, a sensorsuch as a color registration sensor to detect color registration and asensor such as a color toner density (CTD) sensor to detect imagedensity are separate, so color registration error and image densityerror are separately detected and compensated for.

[0020]FIG. 4 is a diagram of a color registration sensor disclosed inU.S. Pat. No. 5,241,400. Referring to FIG. 4, charge coupled devices(CCDs) 40 a and 40 b are provided as color registration sensors. CCDdrivers 48 and 49 are provided to drive the CCDs 40 a and 40 b. Aregistration adjuster 40 registers signals produced by the CCDs 40 a and40 b. A system controller 41 receives a signal from the registrationadjuster 40 and controls a system 45. A mode of a signal output from thesystem controller 41 is converted by a mode switching circuit 42, and isinput into a driver 43. The driver 43 drives the system 45 according tothe input signal.

[0021] The above conventional technique has drawbacks in that a pulsegenerator and a CCD driver are required to drive the CCDs in order toconfigure a system for detecting color registration and image densityusing the CCD, and the configuration of a signal detector is complicatedand difficult since the levels of analog signals detected by the CCDsare different.

[0022]FIG. 5 is a diagram of an image density sensor disclosed in U.S.Pat. No. 6,115,512. Referring to FIG. 5, a white light source 58 toradiate a beam at marks (not shown) on a transfer belt 50 is provided atthe center between light receiving devices 53 and 55. Filters 52 and 54to selectively receive beams reflected from the marks on the transferbelt 50 according to color are provided on the front sides of the lightreceiving devices 55 and 53, respectively. The white light source 58 isa light emitting diode (LED). A beam radiated from the white lightsource 58 should have a wide area in order to sufficiently illuminatethe marks. Accordingly, the detection area of each of the lightreceiving devices 53 and 55 should be wide. Since the spot size of thebeam radiated from the LED white light source 58 used in the aboveconventional technique is large, detection errors in color registrationare large, making it difficult to detect color registration.

[0023]FIG. 6 is a diagram of an image density sensor disclosed in U.S.Pat. No. 5,303,037. Referring to FIG. 6, a red light source 65 isprovided at the center between a blue light source 63 and a green lightsource 67. An acrylic prism 69 directs the beams output by the blue, redand green light sources 63, 65 and 67 into a focusing lens 64 whichfocuses the beams onto a mark (not shown) on a transfer belt 62. Sincethe above-described conventional technique uses the three color lightsources 63, 65, and 67 for an image density sensor, the configuration ofthe sensor is complicated by the need to maintain a constant lightoutput from the light sources 63, 65, and 67, making the sensorexpensive.

[0024] As described above, conventional image forming apparatuses areprovided with separate sensors for color registration and image density.They do not use a sensor performing both functions together. Inaddition, conventional color registration sensors have complicatedconfigurations and poor performance, so it is difficult to accuratelydetect color registration error.

SUMMARY OF THE INVENTION

[0025] Accordingly, it is an object of the present invention to providea color registration control apparatus with a simple structure toaccurately detect color registration, and a method of calculating colorregistration error.

[0026] It is another object of the present invention to provide anapparatus to control color registration and image density, to therebysimultaneously detect color registration and image density.

[0027] Additional objects and advantages of the invention will be setforth in part in the description which follows and, in part, will beobvious from the description, or may be learned by practice of theinvention.

[0028] The foregoing and other objects of the invention are achieved byproviding an apparatus to control color registration in a printer havinga transfer belt. The apparatus includes a plurality of registrationmarks arranged on the transfer belt in a cross-scan direction thereof,parallel to a direction in which the transfer belt moves andperpendicular to a scan direction, each of the registration marksincluding a scan direction component, and a slanting direction componentat an angle with respect to both the scan direction and the cross-scandirection; and a registration sensor provided above the registrationmarks, to radiate beams onto the registration marks in the scandirection so that the beams are perpendicular to the direction in whichthe transfer belt moves, and to receive the radiated beams which arereflected from the registration marks to produce detection signals andto obtain registration information from the detection signals.

[0029] According to an aspect of the present invention, each of theregistration marks has a wedge shape in which the scan directioncomponent is adjacent to the slanting direction component, and theregistration marks are arranged at regular intervals in the cross-scandirection. The registration marks may be arranged on opposite sides ofan image area symmetrically or in the same pattern.

[0030] The scan direction components of registration marks of differentcolors may be arranged at regular intervals in the cross-scan direction.Behind the last scan direction component, the slanting directioncomponents of the registration marks of different colors may be arrangedat regular intervals in the cross-scan direction. The registration marksmay be arranged on opposite sides of an image area in the same pattern.The different colors may be cyan, magenta and yellow, or may be cyan,magenta, yellow and black.

[0031] According to another aspect of the present invention, theregistration marks are of different colors, and the registration sensorincludes an optical module including a light emitter to radiate beamsonto the registration marks, and a light receiver to receive beamsreflected from the registration marks; a light emitter control unit tocontrol the light emitter so that an amount of the beams emitted fromthe light emitter is constant; a color registration control unit toobtain information to compensate for a color registration error fromdetection signals produced by the light receiver; and a system controlunit to receive the information from the color registration control unitto compensate for the color registration error and to control the colorregistration control unit and the printer.

[0032] The light emitter may include a light source to emit beams, and aconverging lens to converge the beams emitted from the light source ontothe registration marks. Furthermore, the light source may be a laserdiode.

[0033] The light receiver may include a photodetector to receive beamsand perform photoelectric conversion, and a converging lens to convergethe beams reflected from the registration marks onto the photodetector.

[0034] According to yet another aspect of the present invention, thelight emitter control unit includes an emitted light measurer to detectthe amount of light emitted from the light emitter, and a light emitterdriver to compare the amount of light detected by the emitted lightmeasurer with a reference amount and to drive the light emitter suchthat the amount of light emitted from the light emitter is constant.

[0035] The color registration control unit may include an amplifier toamplify the detection signals from the light receiver, a mark positiondetector to detect the positions of the registration marks from signalsoutput from the amplifier, and an offset calculator to calculate offsetsfrom the detected positions of the registration marks.

[0036] The amplifier may include a current-to-voltage converter toconvert the detection signals produced by the light receiver fromcurrent to voltage.

[0037] The system control unit may include an offset controller tochange output values of the amplifier to compensate for differences inthe amount of light of the beams reflected from the color registrationmarks of different colors, and a printer controller to receive offsetinformation from the offset calculator and control the printer.

[0038] The foregoing and other objects of the invention are achieved byproviding an apparatus to control color registration and an imagedensity in a printer having a transfer belt and an image density. Theapparatus includes a plurality of registration marks arranged on thetransfer belt in a cross-scan direction thereof, parallel to a directionin which the transfer belt moves and perpendicular to a scan direction,each of the registration marks including a scan direction component, anda slanting direction component at an angle with respect to both the scandirection and the cross-scan direction; a plurality of image densitymarks arranged on the transfer belt in the cross-scan direction, each ofthe image density marks having an image density; and a registration andimage density sensor provided above the registration marks and the imagedensity marks, to radiate beams onto the registration marks and theimage density marks in the scan direction so that the radiated beams areperpendicular to the direction in which the transfer belt moves, and toreceive beams reflected from the registration marks and the imagedensity marks to produce detection signals therefrom and to obtainregistration information and image density information from thedetection signals.

[0039] According to an aspect of the present invention, each of theregistration marks has a wedge shape in which the scan directioncomponent is adjacent to the slanting direction component, and theregistration marks are arranged at regular intervals in the cross-scandirection. The registration marks may be arranged on opposite sides ofan image area symmetrically or in the same pattern.

[0040] According to another aspect of the present invention, the scandirection components of the registration marks of different colors arearranged at regular intervals in the cross-scan direction, and behindthe last scan direction component the slanting direction components ofthe registration marks of different colors are arranged at regularintervals in the cross-scan direction, and the registration marks arearranged on opposite sides of an image area in the same pattern. Thedifferent colors may be cyan, magenta and yellow or may be cyan,magenta, yellow and black.

[0041] According to yet another aspect of the present invention, theregistration and image density marks are of different colors and theregistration and image density sensor includes an optical moduleincluding a light emitter to radiate beams onto the registration marksand the image density marks, and a light receiver to receive beamsreflected from the registration marks and the image density marks; alight emitter control unit to control the light emitter so that anamount of the beams emitted from the light emitter is constant; a colorregistration control unit to obtain information to compensate for acolor registration error from detection signals produced by the lightreceiver; an image density control unit to obtain information tocompensate for an image density error from the detection signalsproduced by the light receiver; and a system control unit to receive theinformation from the color registration control unit to compensate forthe color registration error and the information from the colorregistration control unit to compensate for the image density error andto control the color registration control unit, the image densitycontrol unit, and the printer.

[0042] The light emitter may include a light source to emit light beams;and a converging lens to converge the beams emitted from the lightsource onto the registration marks or the image density marks. The lightsource may be a laser diode.

[0043] The light receiver may include a photodetector to receive thebeams and perform photoelectric conversion, and a converging lens toconverge the beams reflected from the registration marks onto thephotodetector or the image density marks.

[0044] The light emitter control unit may include an emitted lightmeasurer to detect the amount of light emitted from the light emitter,and a light emitter driver to compare the amount of light detected bythe emitted light measurer with a reference amount and drive the lightemitter such that the amount of light emitted from the light emitter ismaintained constant.

[0045] The color registration control unit may include an amplifier toamplify the detection signals from the light receiver, a mark positiondetector to detect the positions of the registration marks from signalsoutput from the amplifier, and an offset calculator to calculate offsetsfrom the detected positions of the registration marks.

[0046] The image density control unit may include an amplifier toamplify the detection signals produced by the light receiver, an imagedensity detector to detect attributes of image density for differentcolors from signals received from the amplifier, and a deviationcalculator to compare the detected attributes with reference attributesand to calculate the deviation therebetween.

[0047] The amplifier may include a current-to-voltage converter toconvert the detection signals produced by the light receiver fromcurrent to voltage.

[0048] According to yet another aspect of the present invention, thesystem control unit includes an offset controller to change outputvalues of the amplifier to compensate for differences in an amount oflight of the beams reflected from the color registration marks or theimage density marks of different colors, and a printer controller toreceive offset information from the offset calculator and the deviationfrom the deviation calculator, and to control the printer.

[0049] The foregoing and other objects of the present invention are alsoachieved by providing a method of calculating color registration errorin an apparatus including a transfer belt. The method includes formingthe registration marks on the transfer belt in a cross-scan directionperpendicular to a scan direction, each of the registration marksincluding a scan direction component and a slanting direction componentat an angle with respect to both the scan direction and the cross-scandirection; radiating beams onto one of the registration marks on thetransfer belt; and receiving beams reflected from the radiatedregistration mark to produce a detection signal; and calculating a colorregistration error based on the detection signal.

[0050] The forming of the registration marks may include forming each ofthe registration marks to have a different color and to have a wedgeshape in which the scan direction component is adjacent to the slantingdirection component.

[0051] The forming of the registration marks includes forming theregistration marks to have different colors; and forming theregistration marks such that the scan direction components and theslanting direction components are arranged at regular intervals in thecross-scan direction.

[0052] The receiving of the beams may include calculating an offseterror in the cross-scan direction from a difference between apredetermined interval between the scan direction components of tworegistration marks of different colors among the color registrationmarks arranged on a same side of the transfer belt, and a detectedinterval therebetween.

[0053] The forming of the registration marks may include forming theregistration marks on opposite sides of an image area of the transferbelt in a same pattern or a symmetrical pattern.

[0054] The forming of the registration marks may include forming theregistration marks on opposite sides of an image area of the transferbelt in a same pattern.

[0055] The receiving of the beams may include calculating an offseterror in the scan direction from a difference between a detectedinterval between the scan direction component of a first one of theregistration marks and the slanting direction component thereof, and adetected interval between the scan direction component of a second oneof the registration marks and the slanting direction component thereof,wherein the first and second registration marks are positioned on thesame side of the transfer belt.

[0056] The receiving of the beams may include calculating a printingwidth error from a difference between a detected interval between thescan direction component of a first one of the registration marks andthe slanting direction component thereof, and a detected intervalbetween the scan direction component of a second one of the registrationmarks and the slanting direction component thereof, wherein the firstand second registration marks are positioned at opposite sides of thetransfer belt, the first and second marks having a same color.

[0057] The receiving of the beams may include calculating a skew from adifference between a detected interval between the scan directioncomponents of two different colored registration marks positioned on afirst side of the transfer belt, and a detected interval between thescan direction components of two different colored registration markspositioned on a second of the transfer belt, wherein the two differentcolored registration marks on the first side have the same colors,respectively, as the two different colored registration marks on thesecond side.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] These and other objects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

[0059]FIG. 1 is a diagram of a conventional color registration sensorand mark pattern;

[0060]FIG. 2 is a diagram of another conventional color registrationsensor and mark pattern;

[0061]FIG. 3 is a diagram of yet another conventional color registrationsensor and mark pattern;

[0062]FIG. 4 is a diagram of still another conventional colorregistration sensor and mark pattern;

[0063]FIG. 5 is a diagram of a conventional image density sensor;

[0064]FIG. 6 is a diagram of another conventional image density sensor;

[0065]FIG. 7 is a block diagram of a color registration controlapparatus according to a first embodiment of the present invention;

[0066]FIG. 8 is a sectional view of the optical module configuration ofa color registration sensor according to the first embodiment of thepresent invention;

[0067]FIG. 9 is a schematic perspective view of a printer in which acolor registration control apparatus according to the first embodimentof the present invention is installed;

[0068]FIG. 10 is a diagram of a beam radiated from the colorregistration sensor according to the present invention;

[0069]FIG. 11 is a schematic diagram of the scattered waveform of a beamwhich is detected by the color registration sensor according to thepresent invention;

[0070]FIG. 12 is a diagram of signals according to color produced by thecolor registration sensor according to a first arrangement ofregistration marks of the present invention;

[0071]FIG. 13 shows offsets calculated by the color registration sensoraccording to the first arrangement of registration marks of the presentinvention;

[0072]FIGS. 14A through 14C show X-offsets calculated by the colorregistration sensor according to the present invention;

[0073]FIG. 15 is a block diagram of an apparatus to control colorregistration and image density according to a second embodiment of thepresent invention;

[0074]FIG. 16 is a diagram of a color registration mark pattern and animage density mark pattern according to the second embodiment of thepresent invention;

[0075]FIG. 17 is a sectional view of a printer in which the apparatus tocontrol color registration and image density according to the secondembodiment of the present invention is installed;

[0076]FIG. 18 is a diagram of signals according to color produced by acolor registration and image density sensor according to a secondarrangement of registration marks of the present invention;

[0077]FIG. 19 shows offsets calculated by the color registration andimage density sensor according to the second arrangement of registrationmarks, of the present invention;

[0078]FIG. 20 shows a color registration signal produced by the colorregistration and image density sensor according to the second embodimentof the present invention; and

[0079]FIG. 21 shows an image density signal produced by the colorregistration and image density sensor according to the second embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0080] Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

[0081] The present invention provides a color registration controlapparatus to accurately detect registration marks to performregistration on different color images, an apparatus to control colorregistration and image density which can simultaneously detect colorregistration and image density, and a method of calculating scandirection, cross-scan direction, printing width, and skew registrationerrors according to color.

[0082] Hereinafter, a color registration control apparatus according tothe present invention will be described in detail with reference to theattached drawings.

[0083]FIG. 7 is a block diagram of a color registration controlapparatus according to an embodiment of the present invention. Referringto FIG. 7, the color registration control apparatus includes aregistration mark 120, and a color registration sensor including anoptical module 101, a light emitter control unit 103, a colorregistration control unit 105, and a system control unit 107.

[0084] The registration mark 120 includes a scan direction component,and a slanting direction component at an angle to the scan andcross-scan directions.

[0085] The optical module 101 includes a light emitter to radiate lightbeams onto the registration mark 120, and a light receiver to receivebeams reflected from the registration mark 120. The light emitterincludes a light source 101-1 to generate and emit the light beams, anda focusing lens 101-2 to focus the beams emitted from the light source101-1 onto the registration mark 120. A laser diode is used as the lightsource 101-1. The light receiver includes a photodetector 101-3 toreceive the beams and perform photoelectric conversion of the receivedbeams, and a focusing lens 101-4 to focus the beams emitted from thelight emitter and reflected from the registration mark 120 onto thephotodetector 101-3.

[0086] The light emitter control unit 103 detects the amount of lightemitted from the light emitter and controls the light emitter tomaintain a constant emission. The light emitter control unit 103includes a first amplifier (AMP) 103-3 to amplify a signal representingan amount of the light emitted from the light source 101-1, an emittedlight measurer 103-1 to receive an output signal of the first AMP 103-3and to measure the amount of the light emitted from the light emitter, asecond AMP 103-4 to amplify an emitted light amount signal output fromthe emitted light measurer 103-1, and a light emitter driver 103-2 tocompare the output signal of the second AMP 103-4 with a referenceemitted light amount signal, and to output the reference emitted lightamount signal to the light emitter to control the amount of the emittedlight.

[0087] A current signal produced by the light receiver is transmitted tothe color registration control unit 105. The color registration controlunit 105 obtains information to compensate for color registration errorsfrom the current signal produced by the light receiver. Here, theinformation to compensate for color registration errors refers to offsetin the scan direction, offset in the cross-scan direction, error inprinting width, and skew.

[0088] The color registration control unit 105 includes acurrent-to-voltage (I/V) converter 105-4 to convert the current signalproduced by the light receiver into a voltage signal, an AMP 105-1 toamplify the voltage signal from the I/V converter 105-4, a low-passfilter (LPF) 105-5 to pass only a low frequency band of the amplifiedsignal, a mark position detector 105-2 to detect the position of theregistration mark 120 from a signal received from the LPF 105-5, and anoffset calculator 105-3 to calculate an offset from the value of thedetected mark position. Here, the offset includes information about anX-offset in the scan direction, a Y-offset in the cross-scan direction,error in printing width, and skew.

[0089] The system control unit 107 includes a printer controller 107-2to receive information to compensate for a color registration error andto control a printer 108, and an offset controller 107-1 to change theoutput value of the AMP 105-1 to compensate for a difference in theamount of light of beams reflected from a pair of the registration marks120 of the same color.

[0090]FIG. 8 is a sectional view of the optical module configuration ofa color registration sensor used in a color registration controlapparatus according to the embodiment of the present invention of FIG.7. Referring to FIG. 8, an optical module 130 is provided with a lightemitter including a laser diode 111 as a light source and a focusinglens 117 to focus beams emitted from the laser diode 111 onto aregistration mark 120 (see FIG. 10) on a transfer belt 110. Acollimating lens 113 to convert beams emitted from the laser diode 111into parallel beams is further provided on the optical path between thelaser diode 111 and the focusing lens 117.

[0091] Referring to FIG. 10, a spot size of a beam radiated onto theregistration mark 120 is no greater than about 200 μm. If the size ofthe spot is decreased to 100 μm or less, detection performance can beimproved. The sensor can be made more reliable if the beam is reflectedonly at a position where it meets the registration mark 120. Inaddition, errors caused by chromatic aberration can be reduced if theemitted beam has a single wavelength.

[0092] The optical module 130 further includes a light receiverincluding a photodetector 115 to receive beams reflected from theregistration mark 120 and perform photoelectric conversion, and afocusing lens 117 provided between the registration mark 120 and thephotodetector 115 to focus beams reflected from the registration mark120 onto the photodetector 115.

[0093] Referring to FIG. 10, when the registration mark 120 shifts, thespot of the beam emitted from the light source shifts, as shown in thedrawing. When the spot of the emitted beam is at the center of theregistration mark 120, a maximum detection signal can be obtained. Foroptimum performance, the light receiver is designed to receive onlybeams diffusely reflected, rather than beams regularly reflected at anangle equal to the angle of incidence, thereby reducing detection error.

[0094]FIG. 9 is a schematic perspective view of a printer in which acolor registration control apparatus according to the embodiment of thepresent invention is installed. Referring to FIG. 9, patterns of theregistration marks 120 are formed on the left and right sides of animage area 126 of the transfer belt 110. Color registration sensors 121are provided above the registration marks 120.

[0095] Here, reference numeral 122 denotes a laser scanning unit (LSU),reference numeral 123 denotes a belt position sensor, reference numeral124 denotes an organic photoconductive cell (OPC) drum, referencenumeral 125 denotes a transfer roller, and reference numeral 128 denotesa transfer belt drive roller.

[0096] Color registration errors may have various causes in an LSU and abelt drive mechanism, and during belt steering and the assembly process.A belt steering error arises from belt weaving or deformation of thebelt unit. An error during the assembly process may arise during theassembly of the OPC drums 124, and the assembly of the LSUs 122.

[0097] An error in the LSU arises from irregular laser scan speed,asynchronization of a polygon mirror (not shown), jitter in an LSU motor(not shown), nonparallel laser beams, and mismatch in bow between laserbeams. Here, asynchronization of a polygon mirror may be caused byinaccurate manufacture or imbalance during horizontal rotation, andcauses an error in a scanning line. When laser beams are not paralleldue to misalignment or mismatch in laser beam bow, toners are developedin the form of a bow, so an error may occur.

[0098] An error which may occur in a belt and OPC drum drive mechanism(not shown) arises from a change in the diameter of a roll due totemperature, a change in the linear velocity of the transfer belt 110due to load on the belt, a change in rotary speed due to load on the OPCdrum 124, and irregular driving of a transfer belt drive roller 128.

[0099]FIG. 11 is a diagram of a waveform of a beam detected by aphotodetector. FIG. 11 is provided to explain a method of detecting theposition of a color registration mark. Referring to FIGS. 10 and 11, itcan be seen from the waveform of a detection signal of beams reflectedfrom a registration mark 120 that the power of the detection signaloutput from a registration sensor (not shown in FIGS. 10 and 11) risesas the registration mark 120 on the transfer belt (not shown in FIGS. 10and 11) approaches the registration sensor, remains constant as theregistration mark 120 passes the center of the registration sensor, andgradually falls as the registration mark 120 moves away from theregistration sensor.

[0100] The time taken for the power to rise from the minimum to themaximum value is represented by T_(rising), and the time taken for thepower to fall from the maximum value to the minimum value is representedby T_(falling). Times T_(rising) and T_(falling) depend on the spot sizeof the beam. As the spot size of the beam is smaller, times T_(rising)and T_(falling) decrease, so that a mark detection error decreases.

[0101] Here, the position W of the mark 120 is determined by Formula(1). T_(width) indicates the time between the middle of the timeT_(rising) and the middle of the time T_(falling).

W=T _(width)/2   (1)

[0102]FIG. 12 is a diagram of signals produced with respect toregistration marks of different colors and masked by a colorregistration sensor according to the embodiment of the presentinvention. Referring to FIG. 12, it can be seen from a graph of a firstdetection signal that the first detection signal output from the colorregistration sensor includes a scan direction signal component and aslanting direction signal component at the position of a first colorregistration mark 120-1. Masking is performed to prevent signals ofsecond through fourth color registration marks 120-2, 120-3, and 120-4from being produced. The graphs of second through fourth detectionsignals of the second through fourth color registration marks 120-2,120-3, and 120-4 can be explained in the same manner as the graph of thefirst detection signal.

[0103] Here, T_(y2) indicates the time interval between the scandirection component of the first color registration mark 120-1 and thescan direction component of the second color registration mark 120-2.T_(y3) indicates the time interval between the scan direction componentof the first color registration mark 120-1 and the scan directioncomponent of the third color registration mark 120-3. T_(y4) indicatesthe time interval between the scan direction component of the firstcolor registration mark 120-1 and the scan direction component of thefourth color registration mark 120-4.

[0104]FIG. 13 illustrates a method of calculating color registrationerror and image density. The method uses an apparatus which includesregistration marks and image density marks arranged in a cross-scandirection parallel to the moving direction of a transfer belt of aprinter. A registration sensor and an image density sensor providedabove the registration marks and the image density marks, respectively,radiate beams in a scan direction perpendicular to the moving directionof the transfer belt. The method of calculating color registrationerrors includes (1) forming the registration marks, each having a scandirection component and a slanting direction component at an angle withrespect to the scan direction and the cross-scan direction; (2)radiating beams onto the registration marks on the transfer belt, and(3) calculating color registration errors using signals produced frombeams reflected from the registration marks.

[0105] In operation (1), each registration mark is formed such that thescan direction component and the slanting direction component areadjacent, to form the shape of a wedge, and the scan directioncomponents and the slanting direction components of the registrationmarks of different colors are alternately arranged at opposite sides ofan image area at regular intervals in the cross-scan direction. The scanand slanting direction components of the registration marks of thedifferent colors may be arranged symmetrically about a center of thetransfer belt or asymmetrically to form a same pattern on each side.Alternatively, in operation (1), the scan direction components of theregistration marks of the different colors may be arranged at theopposite sides of the image area at regular intervals, and then behindthe last scan direction component the slanting direction components ofthe registration marks of the different colors are arranged at regularintervals so that the registration marks at the opposite sides of theimage area are formed asymmetrically in the same pattern.

[0106] First through fourth color registration mark pairs 120-5 through120-8 formed on the left and right sides of a transfer belt (not shownin FIG. 13) are shown in the upper portion of FIG. 13, and methods ofcalculating X-offsets, Y-offsets, printing width errors, and skew withrespect to the second through fourth color registration mark pairs 120-5through 120-8 are shown in the lower portion of FIG. 13.

[0107] X-offset, that is, scan direction error, with respect to colorregistration marks can be obtained from the differences between timeintervals between the scan direction components and the slantingdirection components of the respective color registration marks.

[0108] An X-offset with respect to the second color registration mark onthe left side is expressed by Formula (2). Here, T_(xs1) indicates thetime interval between the scan direction component of the first colorregistration mark on the left side and the slanting component thereof,and T_(xs2), T_(xs3), and T_(xs4) indicate the same time interval withrespect to the second, third and fourth color registration marks,respectively, on the left side.

T_(xs1)−T_(xs2)   ( 2 )

[0109] When Formula (2) gives a negative result, T_(xs2) is greater thanT_(xs1), which means that the second color registration mark on the leftside is positioned further to the left than the first color registrationmark on the left side. In this case, scan direction error can be reducedby increasing the X-offset. When Formula (2) gives a positive result,T_(xs2) is less than T_(xs1) which means that the second colorregistration mark on the left side is positioned further to the rightthan the first color registration mark on the left side. In this case,scan direction error can be reduced by decreasing the X-offset.

[0110] X-offsets of the third and fourth color registration marks on theleft can be described in the same manner. The X-offset of the thirdcolor registration mark on the left is expressed by Formula (3), and theX-offset of the fourth color registration mark on the left is expressedby Formula (4).

T_(xs1)−T_(xs3)   (3)

T_(xs1)−T_(xs4)   (4)

[0111] The same principles can be applied to the second through fourthregistration marks on the right.

[0112] Y-offset, that is, cross-scan direction error, of colorregistration marks are calculated from the difference betweenpredetermined time intervals between the scan direction components ofthe respective color registration marks of different colors arranged ina cross-scan direction and detected time intervals therebetween.

[0113] A Y-offset of the second color registration mark on the left isthe difference between T_(y2) (shown in FIG. 12) and T_(ys12) (shown inFIG. 13), and is expressed by Formula (5). Here, T_(ys12) indicates adetected time interval between the scan direction component of the firstcolor registration mark on the left and the scan direction component ofthe second color registration mark on the left. T_(ys12) is apredetermined value, but T_(ys12) is a variable.

T_(y2)−T_(ys12)   (5)

[0114] When the Y-offset is negative, T_(ys12) is greater than T_(ys2),that is, the detected time interval is longer than the predeterminedtime interval. This means that a page is delayed. Accordingly,cross-scan direction error can be reduced by advancing the page. Whenthe Y-offset is positive, it can be inferred that a page is advancedbased on the above principle. Accordingly, cross-scan direction errorcan be reduced by delaying the page.

[0115] Y-offset of the third and fourth color registration marks on theleft can be described based on the same principles as described above.The Y-offset of the third registration mark on the left is expressed byFormula (6), and the Y-offset of the fourth registration mark on theleft is expressed by Formula (7).

T_(y3)−T_(ys13)   (6)

T_(y4)−T_(ys14)   (7)

[0116] The same principles can be applied to the second through fourthregistration marks on the right.

[0117] Printing width error can be obtained from the difference betweena first differential value and a second differential value. Each of thefirst and second differential values is the difference between the timeinterval between the scan direction component and the slanting directioncomponent of a color registration mark on the left, and the timeinterval between the scan direction component and the slanting directioncomponent of a color registration mark of the same color on the right.

[0118] A printing width error of the second color registration mark pair120-6 is expressed by Formula (8).

(T_(xs1)−T_(xe1))−(T_(xs2)−T_(xe2))   (8)

[0119] When Formula (8) gives a negative result, the printing widthbetween the second left and right registration marks is greater than theprinting width between the first left and right registration marks. Inthis case, reduction of the printing width is required. When Formula (8)gives a positive result, the opposite is true. The same principles asdescribed above can be applied to printing width errors of the third andfourth left and right color registration marks. Here, T_(xe1) indicatesthe detected time interval between the scan direction component and theslanting direction component of the first color registration mark on theright, and T_(xe2), T_(xe3), and T_(xe4) indicate the same timeintervals with respect to the second through fourth color registrationmarks on the right.

[0120] Printing width error of the third left and right colorregistration marks is expressed by Formula (9), and printing width errorof the fourth left and right color registration marks is expressed byFormula (10).

(T_(xs1)−T_(xe1))−(T_(xs3)−T_(xe3))   (9)

(T_(xs1)−T_(xe1))−(T_(xs4)−T_(xe4))   (10)

[0121] Skew can be obtained from the difference between a detected timeinterval between the scan direction components of two different colorregistration marks arranged in a cross-scan direction on the left, and adetected time interval between the scan direction components ofcorresponding two different color registration marks arranged in across-scan direction on the right.

[0122] Skew with respect to the second left and right color registrationmarks is expressed by Formula (11). Even when the above three kinds oferrors do not arise, an error in a polygon mirror in an LSU (not shownin FIG. 13) or a laser scan error may cause a scanning line to skew.

T_(ys12)−T_(ye12)   (11)

[0123] When Formula (11) gives a negative result, T_(ye12) is greaterthan T_(ys12), representing skew to the right. When Formula (11) gives apositive result, skew is to the left. Here, T_(ys12) indicates the timeinterval between the scan direction components of the first and secondcolor registration marks on the left, T_(ye12) indicates the timeinterval between the scan direction components of the first and secondcolor registration marks on the right, T_(ys13) indicates the timeinterval between the scan direction components of the first and thirdcolor registration marks on the left, T_(ye13) indicates the timeinterval between the scan direction components of the first and thirdcolor registration marks on the right, T_(ys14) indicates the timeinterval between the scan direction components of the first and fourthcolor registration marks on the left, and T_(ye14) indicates the timeinterval between the scan direction components of the first and fourthcolor registration marks on the right. Skew with respect to the thirdand fourth color registration mark pairs 120-7 and 120-8 is expressed byFormula (12) and Formula (13), respectively.

T_(ys13)−T_(ye13)   (12)

T_(ys14)−T_(ye14)   (13)

[0124] The above methods of calculating X-offset, Y-offset, printingwidth error and skew can be applied to a method of controlling a printerhaving registration marks for different colors, which each includes onlya scan direction component and they are arranged on the left and rightsides of a transfer belt in a cross-scan direction.

[0125] In a method of controlling a printer having registration marksfor different colors, which are arranged only on one side of a transferbelt in a cross-scan direction, the above method of calculating Y offsetcan be used.

[0126] A registration mark can include a scan direction component and aslanting direction component in a wedge shape, and a plurality of theregistration marks can be arranged in the same pattern or in asymmetrical pattern.

[0127] As shown in FIG. 19, a mark pattern, in which scan directioncomponents for first through fourth colors are arranged at regularintervals and slanting direction components for the first through fourthcolors are arranged at regular intervals behind the scan directioncomponent for the fourth color, can be formed in the same manner on eachof the left and right sides.

[0128]FIGS. 14A through 14C show signals produced when the X-offset is10 dots, when the X-offset is 50 dots, and when the X-offset is 100dots, respectively. Referring to FIGS. 14A through 14C, since each offirst through fourth color registration marks includes a scan directioncomponent and a slanting direction component, two adjacent points atwhich the output voltage suddenly rises form a pair. In the case wherethe fourth color is black, a fourth color registration mark is formed bydeveloping a color other than black and forming a color registrationmark on the developed color in a wedge shape. Since a beam is absorbedby the black fourth color registration mark, the signal falls at thepoint where the fourth color registration mark is detected.

[0129] It can be seen that as the X-offset increases from 10 dots to 50dots to 100 dots, the interval between the time when the scan directioncomponent of a color registration mark is detected and the time when theslanting direction component thereof is detected gets greater. In otherwords, the degree of X-offset can be estimated by measuring the timeinterval between adjacent rising points in an offset detection signal.Accordingly, information used to control a printer can be obtained.

[0130] In the above-described apparatus and method to detectregistration marks, since the spot size is reduced so that a beam can bereflected from only a registration mark, probability and reliability ofdetection of the registration mark can be increased. Since onlydiffusely reflected components are detected, detection error can bereduced. In addition, radiation of a beam having a single wavelength canremove chromatic aberration. Since color registration sensors areprovided on the left and right sides, the above four kinds of errors canbe simultaneously detected.

[0131] However, since the above color registration sensor is for colorregistration, a separate sensor to detect an image density is required.Accordingly, in addition to a color registration sensor and a method ofcalculating a color registration error, the present invention alsoprovides a sensor to simultaneously detect color registration and imagedensity and a method of calculating color registration error and imagedensity error using the sensor.

[0132] Hereinafter, a color registration and image density sensoraccording to the present invention will be described in detail withreference to the attached drawings.

[0133]FIG. 15 is a block diagram of an apparatus to control colorregistration and image density according to another embodiment of thepresent invention. Referring to FIG. 15, the apparatus includes twocolor registration and image density sensors provided on the left andright sides and a pair of color registration mark patterns or two imagedensity mark patterns provided on the left and right sides.

[0134] A first registration and image density sensor includes a firstoptical module 201, a first light emitter control unit 203, a firstcolor registration control unit 205, a first image density control unit206, and a system control unit 207. A second registration and imagedensity sensor includes a second optical module 202, a second lightemitter control unit 204, a second color registration control unit 209,a second image density control unit 210, and the system control unit207.

[0135] The first and second optical modules 201 and 202 include lightemitters to radiate beams onto first and second mark patterns 220 and222, respectively, and light receivers to receive beams reflected fromthe first and second mark patterns 220 and 222, respectively. The lightemitters include light sources 201-1 and 202-1, respectively, togenerate and emit light beams, and focusing lenses 201-2 and 202-2,respectively, to focus the beams emitted from the respective lightsources 201-1 and 202-1 onto the first and second mark patterns 220 and222, respectively. Laser diodes are used as the light sources 201-1 and202-1.

[0136] The light receivers include photodetectors 201-3 and 202-3,respectively, to receive the emitted beams and perform photoelectricconversion, and focusing lenses 201-4 and 202-4, respectively, to focusthe light beams emitted from the respective light emitters and reflectedfrom the respective first and second mark patterns 220 and 222 onto thephotodetectors 201-3 and 202-3, respectively.

[0137] The first and second light emitter control units 203 and 204detect the amount of light emitted from the respective light emittersand control the light emitters to maintain a constant emission. Each ofthe first and second light emitter control units 203 and 204 includes afirst AMP 203-3 or 204-3 to amplify a signal representing the amount oflight of beams emitted from the light source 201-1 or 202-1, and anemitted light measurer 203-1 or 204-1 to receive an output signal of thefirst AMP 203-3 or 204-3 and measure the amount of light emitted fromeach of the light emitters. The first and second light emitter controlunits 203 and 204 each further include a second AMP 203-4 or 204-4 toamplify an emitted light amount signal output from the emitted lightmeasurer 203-1 or 204-1, and a light emitter driver 203-2 or 204-2 toreceive the output signal of the second AMP 203-4 or 204-4 and tocontrol the amount of light emitted from each of the light emitters.

[0138] Current signals produced by the respective light receivers aretransmitted to the first and second color registration control units 205and 209, respectively, and to the first and second image density controlunits 206 and 210, respectively. The first and second color registrationcontrol units 205 and 209 obtain information to compensate for colorregistration errors from the current signal produced by the respectivelight receivers.

[0139] The first and second color registration control units 205 and 209include I/V converters 205-4 and 209-4 to convert the current signalsproduced by the respective light receivers into voltage signals, AMPs205-1 and 209-1 to amplify the voltage signals from the respective I/Vconverters 205-4 and 209-4, LPFs 205-5 and 209-5 to pass only lowfrequency bands of the respective amplified signals, mark positiondetectors 205-2 and 209-2 to detect the positions of the first andsecond mark patterns 220 and 222 from signals received from therespective LPFs 205-5 and 209-5, and offset calculators 205-3 and 209-3to calculate offsets from the values of the respective detected markpositions. Here, the offsets include information about X-offset,Y-offset, printing width error, and skew.

[0140] The first and second image density control units 206 and 210include I/V converters 206-4 and 210-4 to convert the current signalsproduced by the respective light receivers into voltage signals, AMPs206-1 and 210-1 to amplify the voltage signals from the respective I/Vconverters 206-4 and 210-4, LPFs 206-5 and 210-5 to pass only lowfrequency bands of the respective amplified signals, image densitydetectors 206-2 and 210-2 to detect image density attributes fordifferent colors from output signals of the respective LPFs 206-5 and210-5, and deviation calculators 206-3 and 210-3 to compare the detectedimage density attributes with reference image density attributes and tocalculate the deviation.

[0141] The system control unit 207 includes a printer controller 207-2to receive information to compensate for color registration error andimage density error from the first and second color registration controlunits 205 and 209 and the first and second image density control units206 and 210, and to control a printer 208, and an offset controller207-1 to change the output values of the AMPs 205-1 and 206-1 tocompensate for a difference in the amount of light of beams reflectedfrom the first and second mark patterns 220 and 222. The system controlunit 207 also includes an offset controller 207-3 to change the outputvalues of the AMPs 209-1 and 210-1 to compensate for a difference in theamount of light of beams reflected from the first and second markpatterns 220 and 222.

[0142]FIG. 16 shows an image density mark pattern and a registrationmark pattern formed on a transfer belt of an apparatus to control colorregistration and an image density according to the second embodiment ofthe present invention. Referring to FIG. 16, first through third imageareas 224-1, 224-2, and 224-3 are disposed in the middle of a transferbelt 240. An image density mark pattern 272 and a registration markpattern 270 are arranged in a cross-scan direction on each of the rightand left sides of the transfer belt 240.

[0143] Color registration and image density sensors 221 and 223 areprovided above the transfer belt 240. Each of the registration and imagedensity sensors 221 and 223 radiates a beam onto a portion of the imagedensity mark pattern 272 or the registration mark pattern 270 when theimage density mark pattern 272 or the registration mark pattern 270passes the sensor as the transfer belt 240 moves in a cross-scandirection and produces a detection signal.

[0144]FIG. 17 is a sectional view of a printer in which an apparatus tocontrol color registration and an image density according to the secondembodiment of the present invention is installed. Referring to FIG. 17,a color registration and image density sensor 250 (identical to sensors221 and 223) is provided between an LSU 258 and a transfer roll 251. Atof/weaving sensor 257 is provided between a charger (not shown) and theLSU 258. Here, reference numeral 253 denotes a belt drive roll,reference numeral 255 denotes a dry/fixing device, and reference numeral252 denotes an intermediate transfer belt.

[0145]FIG. 18 is a graph of signals produced for registration marks andimage density marks by a color registration and image density sensoraccording to the second embodiment of the present invention. In a graphof a signal produced by a color registration sensor for a first colorregistration mark, it can be seen that the signal includes a scandirection component pulse and a slanting direction component pulse atpositions corresponding to the first color registration mark. Graphs ofsignals produced for second through fourth color registration marks canbe described in the same manner as the graph of the signal produced forthe first color registration mark.

[0146] Unlike the color registration marks shown in FIG. 12, the scandirection component and the slanting direction component of the samecolor registration mark are not sequential in FIG. 18. In FIG. 18, thescan direction components of the respective first through fourth colorregistration marks appear sequentially, and then the slanting directioncomponents of the respective first through fourth color registrationmarks appear.

[0147] Here, T_(y2) indicates the time interval between the scandirection components of the respective first and second colorregistration marks. T_(y3) indicates the time interval between the scandirection components of the respective first and third colorregistration marks. T_(y4) indicates the time interval between the scandirection components of the respective first and fourth colorregistration marks.

[0148] For image density marks, marks having a grey level of 10% forfirst through fourth colors are arranged in line to thus form a unitset, and consecutively, a set of marks having a grey level of 20% forthe first through fourth colors are arranged in line. With such anarrangement, sets of image density marks for the first through fourthcolors having grey levels of 10 through 100%, increasing in steps of10%, are arranged.

[0149] In FIG. 18, image density signals produced by the colorregistration and image density sensor are sequentially illustrated withrespect to the image density marks for the first through fourth colors.As shown in FIG. 18, when the grey level of an image density mark is100%, the power of the detection signal is greatest. The power of adetection signal decreases as the grey level decreases.

[0150]FIG. 19 shows methods of calculating color registration errors.First through fourth color registration marks formed on the left andright sides of a transfer belt are shown in the upper portion of FIG.19, and methods of calculating X-offset, Y-offset, printing width error,and skew with respect to the second through fourth color registrationmarks are shown in the lower portion of FIG. 19.

[0151] The arrangement of registration marks shown in FIG. 19 isdifferent from that shown in FIG. 13, but the method of calculatingX-offset, Y-offset, printing width error, and skew shown in FIG. 19 isthe same as that shown in FIG. 13. Thus, a repeat description will beomitted.

[0152]FIG. 20 is a graph of the result of amplifying a signal producedwith respect to registration marks. Unlike FIG. 14, a signal withrespect to a color registration mark for the same color does not appearas a pulse pair but as a single pulse. An offset can be calculated fromthe time interval between pulses.

[0153]FIG. 21 is a graph of a signal produced with respect to imagedensity marks. It can be seen that image density, i.e., output voltage,with respect to a grey level, gradually increases as the grey levelincreases. An error can be calculated by comparing the output voltage ofthe image density detection signal produced in a printer with a graph ofa reference image density detection signal.

[0154] A method of calculating errors using a color registration andimage density sensor according to the present invention further includescalculating an image density error by comparing the output voltagedetected with respect to each grey level of an image density of eachcolor with a reference value, in addition to a method of calculatingcolor registration errors.

[0155] According to an apparatus to control color registration and imagedensity and a method of calculating color registration error and imagedensity error according to the embodiments of the present invention,color registration and image density can be detected using a singlesensor. X-offset, Y-offset, printing width error, and skew can besimultaneously detected and used to compensate for registration error.In addition, the image density of each color can be detected.

[0156] Although a few preferred embodiments of the present inventionhave been shown and described, it will be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents. For example, thoseskilled in the art can change the form of a registration mark patternwhen configuring the apparatus to control registration, withoutdeparting from the spirit and scope of the present invention.

[0157] As described above, in an apparatus to control color registrationand a method of calculating registration error according to the presentinvention, four kinds of offset can be simultaneously detected usingregistration marks including a scan direction component and a slantingdirection component, and registration error can be compensated for basedon the detected offsets, thereby reducing detected error.

[0158] In an apparatus to control color registration and an imagedensity according to the present invention, both color registration andimage density can be detected by a single sensor, so X-offset, Y-offset,printing width error, skew, and image density error can besimultaneously compensated for.

What is claimed is:
 1. An apparatus to control color registration in aprinter having a transfer belt, the apparatus comprising: a plurality ofregistration marks arranged on the transfer belt in a cross-scandirection parallel to a direction in which the transfer belt moves, andperpendicular to a scan direction, each of the registration markscomprising: a scan direction component, and a slanting directioncomponent at an angle with respect to both the scan direction and thecross-scan direction; and a registration sensor provided above theregistration marks, to radiate beams onto the registration marks in thescan direction so that the radiated beams are perpendicular to thedirection in which the transfer belt moves, and to receive radiatedbeams which are reflected from the registration marks to producedetection signals in accordance with the received beams and to obtainregistration information from the detection signals.
 2. The apparatus ofclaim 1, wherein each of the registration marks has a wedge shape inwhich the scan direction component is adjacent to the slanting directioncomponent.
 3. The apparatus of claim 2, wherein the registration marksare of different colors and are arranged at regular intervals in thecross-scan direction.
 4. The apparatus of claim 3, wherein theregistration marks are arranged on opposite sides of an image area ofthe transfer belt in a same pattern.
 5. The apparatus of claim 3,wherein the registration marks are symmetrically arranged on oppositesides of an image area of the transfer belt.
 6. The apparatus of claim1, wherein the registration marks are of different colors and the scanand slanting direction components of the registration marks are arrangedat regular intervals in the cross-scan direction, the slanting directioncomponents being displaced behind the scan direction components in thedirection in which the transfer belt moves.
 7. The apparatus of claim 6,wherein the registration marks are arranged on opposite sides of animage area of the transfer belt in a same pattern.
 8. The apparatus ofclaim 3, wherein the different colors are cyan, magenta and yellow. 9.The apparatus of claim 6, wherein the different colors are cyan, magentaand yellow.
 10. The apparatus of claim 3, wherein the different colorsare cyan, magenta, yellow and black.
 11. The apparatus of claim 6,wherein the different colors are cyan, magenta, yellow and black. 12.The apparatus of claim 1, wherein the registration marks are ofdifferent colors, and the registration sensor comprises: an opticalmodule comprising: a light emitter to radiate the beams onto theregistration marks, and a light receiver to receive the radiated beamswhich are reflected from the registration marks and produce thedetection signals; a light emitter control unit to control the lightemitter so that an amount of the beams emitted from the light emitter isconstant; a color registration control unit to obtain information tocompensate for a color registration error from the detection signalsproduced by the light receiver; and a system control unit to receive theinformation from the color registration control unit to compensate forthe color registration error and to control the color registrationcontrol unit and the printer.
 13. The apparatus of claim 12, wherein thelight emitter comprises: a light source to emit the beams; and aconverging lens to converge the beams emitted from the light source ontothe registration marks.
 14. The apparatus of claim 13, wherein the lightsource is a laser diode.
 15. The apparatus of claim 14, wherein thelight receiver comprises: a photodetector to receive the emitted beamsand to perform photoelectric conversion of the received beams; and aconverging lens to converge the beams reflected from the registrationmarks onto the photodetector.
 16. The apparatus of claim 15, wherein thelight emitter control unit comprises: an emitted light measurer todetect the amount of the beams emitted from the light emitter; and alight emitter driver to compare the amount of the beams detected by theemitted light measurer with a reference amount and to drive the lightemitter such that the amount of the beams emitted from the light emitteris maintained constant.
 17. The apparatus of claim 16, wherein the colorregistration control unit comprises: an amplifier to amplify thedetection signals produced by the light receiver; a mark positiondetector to detect positions of the registration marks from signalsoutput from the amplifier; and an offset calculator to calculate offsetsfrom the detected positions of the registration marks.
 18. The apparatusof claim 17, wherein the amplifier comprises a current-to-voltageconverter to convert the detection signals produced by the lightreceiver from current signals to voltage signals.
 19. The apparatus ofclaim 18, wherein the system control unit comprises: an offsetcontroller to change values of the signals output from the amplifier tocompensate for differences in an amount of light of the beams reflectedfrom the registration marks of the different colors; and a printercontroller to receive offset information from the offset calculator andthereby control the printer.
 20. An apparatus to control colorregistration and an image density in a printer having a transfer belt,comprising: a plurality of registration marks arranged on the transferbelt in a cross-scan direction parallel to a direction in which thetransfer belt moves, and perpendicular to a scan direction, each of theregistration marks comprising: a scan direction component, and aslanting direction component at an angle with respect to both the scandirection and the cross-scan direction; a plurality of image densitymarks arranged on the transfer belt in the cross-scan direction, each ofthe image density marks having an image density; and a registration andimage density sensor provided above the registration marks and the imagedensity marks, to radiate beams onto the registration marks and theimage density marks in the scan direction so that the radiated beams areperpendicular to the direction in which the transfer belt moves, and toreceive radiated beams reflected from the registration marks and theimage density marks to produce detection signals in accordance with thereceived beams and to obtain registration information and image densityinformation from the detection signals.
 21. The apparatus of claim 20,wherein each of the registration marks has a wedge shape in which thescan direction component is adjacent to the slanting directioncomponent.
 22. The apparatus of claim 21, wherein the registration marksare of different colors and are arranged at regular intervals in thecross-scan direction.
 23. The apparatus of claim 22, wherein theregistration marks are arranged on opposite sides of an image area ofthe transfer belt in a same pattern.
 24. The apparatus of claim 23,wherein the registration marks are symmetrically arranged on oppositesides of an image area of the transfer belt.
 25. The apparatus of claim21, wherein the registration marks are of different colors and the scandirection and the slanting direction components of the registrationmarks are arranged at regular intervals in the cross-scan direction, theslanting direction components being displaced behind the scan directioncomponents in the direction in which the transfer belt moves.
 26. Theapparatus of claim 25, wherein the registration marks are arranged onopposite sides of an image area of the transfer belt in a same pattern.27. The apparatus of claim 23, wherein the different colors are cyan,magenta and yellow.
 28. The apparatus of claim 25, wherein the differentcolors are cyan, magenta and yellow.
 29. The apparatus of claim 23,wherein the different colors are cyan, magenta, yellow and black. 30.The apparatus of claim 25, wherein the different colors are cyan,magenta, yellow and black.
 31. The apparatus of claim 20, wherein theimage density marks are of different colors and have different imagedensities and are arranged in order according to the image densities.32. The apparatus of claim 31, wherein the image density marks arearranged on first and second sides of an image area of the transfer beltin a same pattern.
 33. The apparatus of claim 20, wherein theregistration and image density marks are of different colors and theregistration and image density sensor comprises: an optical modulecomprising: a light emitter to radiate the beams onto the registrationmarks and the image density marks, and a light receiver to receive theradiated beams which are reflected from the registration marks and theimage density marks and to produce the detection signals; a lightemitter control unit to control the light emitter so that an amount ofthe beams emitted from the light emitter is constant; a colorregistration control unit to obtain information to compensate for acolor registration error from the detection signals produced by thelight receiver; an image density control unit to obtain information tocompensate for an image density error from the detection signalsproduced by the light receiver; and a system control unit to receive theinformation from the color registration control unit to compensate forthe color registration error and the information from the colorregistration control unit to compensate for the image density error andto control the color registration control unit, the image densitycontrol unit, and the printer.
 34. The apparatus of claim 33, whereinthe light emitter comprises: a light source to emit the beams; and aconverging lens to converge the beams emitted from the light source ontothe registration marks or the image density marks.
 35. The apparatus ofclaim 34, wherein the light source is a laser diode.
 36. The apparatusof claim 35, wherein the light receiver comprises: a photodetector toreceive the emitted beams and to perform photoelectric conversion of thereceived beams; and a converging lens to converge the beams reflectedfrom the registration marks or the image density marks onto thephotodetector.
 37. The apparatus of claim 36, wherein the light emittercontrol unit comprises: an emitted light measurer to detect the amountof the beams emitted from the light emitter; and a light emitter driverto compare the amount of the beams detected by the emitted lightmeasurer with a reference amount and to drive the light emitter suchthat the amount of the beams emitted from the light emitter ismaintained constant.
 38. The apparatus of claim 37, wherein the colorregistration control unit comprises: a first amplifier to amplify thedetection signals produced by the light receiver; a mark positiondetector to detect positions of the registration marks from signalsoutput from the first amplifier; and an offset calculator to calculateoffsets from the detected positions of the registration marks.
 39. Theapparatus of claim 38, wherein the image density control unit comprises:a second amplifier to amplify the detection signals produced by thelight receiver; an image density detector to detect attributes of theimage densities of the different colors from signals received from thesecond amplifier; and a deviation calculator to compare the detectedattributes with reference attributes and calculate a deviationtherebetween.
 40. The apparatus of claim 39, wherein the first andsecond amplifiers each comprises a current-to-voltage converter toconvert the detection signals produced by the light receiver fromcurrent signals to voltage signals.
 41. The apparatus of claim 40,wherein the system control unit comprises: an offset controller tochange values of the signals output from the first and second amplifiersto compensate for differences in an amount of light of the beamsreflected from the registration marks or the image density marks of thedifferent colors; and a printer controller to receive offset informationfrom the offset calculator and the deviation from the deviationcalculator and thereby control the printer.
 42. A method of calculatingcolor registration error in an apparatus comprising a transfer belt, themethod comprising: forming registration marks on the transfer belt in across-scan direction perpendicular to a scan direction, each of theregistration marks including a scan direction component and a slantingdirection component at an angle with respect to both the scan directionand the cross-scan direction; radiating beams onto one of theregistration marks on the transfer belt; and receiving radiated beamsreflected from the one of the registration marks to produce a detectionsignal; and calculating a color registration error based on thedetection signal.
 43. The calculating method of claim 42, wherein theforming of the registration marks comprises forming each of theregistration marks to have a different color and to have a wedge shapein which the scan direction component is adjacent to the slantingdirection component.
 44. The calculating method of claim 42, wherein theforming of the registration marks comprises: forming the registrationmarks to have different colors; and forming the registration marks suchthat the scan direction components and the slanting direction componentsare arranged at regular intervals in the cross-scan direction.
 45. Thecalculating method of claim 43, wherein the receiving of the beamscomprises calculating an offset error in the cross-scan direction from adifference between a predetermined interval between the scan directioncomponents of two of the registration marks of different colors amongthe color registration marks arranged on a same side of the transferbelt, and a detected interval therebetween.
 46. The calculating methodof claim 44, wherein the receiving of the beams comprises calculating anoffset error in the cross-scan direction from a difference between apredetermined interval between the scan direction components of two ofthe registration marks of different colors among the color registrationmarks arranged on a same side of the transfer belt, and a detectedinterval therebetween.
 47. The calculating method of claim 42, whereinthe forming of the registration marks comprises forming the registrationmarks on opposite sides of an image area of the transfer belt in a samepattern or a symmetrical pattern.
 48. The calculating method of claim43, wherein the forming of the registration marks comprises forming theregistration marks on opposite sides of an image area of the transferbelt in a same pattern.
 49. The calculating method of claim 47, whereinthe receiving of the beams comprises calculating an offset error in thescan direction from a difference between a detected interval between thescan direction component of a first one of the registration marks andthe slanting direction component thereof, and a detected intervalbetween the scan direction component of a second one of the registrationmarks and the slanting direction component thereof, wherein the firstand second registration marks are positioned on the same side of thetransfer belt.
 50. The calculating method of claim 48, wherein thereceiving of the beams comprises calculating an offset error in the scandirection from a difference between a detected interval between the scandirection component of a first one of the registration marks and theslanting direction component thereof, and a detected interval betweenthe scan direction component of a second one of the registration marksand the slanting direction component thereof, wherein the first andsecond registration marks are positioned on the same side of thetransfer belt.
 51. The calculating method of claim 47, wherein thereceiving of the beams comprises calculating a printing width error froma difference between a detected interval between the scan directioncomponent of a first one of the registration marks and the slantingdirection component thereof, and a detected interval between the scandirection component of a second one of the registration marks and theslanting direction component thereof, wherein the first and secondregistration marks are positioned at opposite sides of the transferbelt, the first and second marks having a same color.
 52. Thecalculating method of claim 48, wherein the receiving of the beamscomprises calculating a printing width error from a difference between adetected interval between the scan direction component of a first one ofthe registration marks and the slanting direction component thereof, anda detected interval between the scan direction component of a second oneof the registration marks and the slanting direction component thereof,wherein the first and second registration marks are positioned atopposite sides of the transfer belt, the first and second marks having asame color.
 53. The calculating method of claim 47, wherein thereceiving of the beams comprises calculating a skew from a differencebetween a detected interval between the scan direction components of twodifferent colored registration marks positioned on a first side of thetransfer belt, and a detected interval between the scan directioncomponents of two different colored registration marks positioned on asecond side of the transfer belt, wherein the two different coloredregistration marks on the first side have the same colors, respectively,as the two different colored registration marks on the second side. 54.The calculating method of claim 48, wherein the receiving of the beamscomprises calculating a skew from a difference between a detectedinterval between the scan direction components of two different coloredregistration marks positioned on a first side of the transfer belt, anda detected interval between the scan direction components of twodifferent colored registration marks positioned on a second side of thetransfer belt, wherein the two different colored registration marks onthe first side have the same colors, respectively, as the two differentcolored registration marks on the second side.
 55. An apparatuscomprising: a belt moving in a first direction perpendicular to a seconddirection; a plurality of marks on the belt, each of the markscomprising: a first component at an angle with respect to the seconddirection, and a second component in the second direction; and a sensorto simultaneously detect an offset of the marks in the first and seconddirections.
 56. The apparatus of claim 55, wherein the sensorsimultaneously detects a skew and a printing width error of the marks.57. The apparatus of claim 55, wherein the sensor radiates beams ontothe marks in the second direction, and receives the radiated beams whichare reflected from the marks to detect the offsets based upon thereflected beams.
 58. The apparatus of claim 57, wherein the beams have asingle wavelength.
 59. The apparatus of claim 57, wherein a spot size ofthe beams on the marks is less than 200 microns.
 60. The apparatus ofclaim 55, wherein the second components alternate with the firstcomponents.
 61. The apparatus of claim 55, wherein the second componentsare arranged in series, and the first components are arranged in series.62. The apparatus of claim 55, wherein the belt comprises first andsecond sides, and the apparatus further comprises first and secondsensors corresponding to the first and second sides, respectively. 63.An apparatus comprising: a belt moving in a first directionperpendicular to a second direction; a plurality of registration markson the belt, each of the registration marks comprising: a firstcomponent at an angle with respect to the second direction, and a secondcomponent in the second direction; a plurality of image density marksarranged on the belt in the first direction; and a sensor tosimultaneously detect registration information and image densityinformation from the registration marks and the image density marks. 64.The apparatus of claim 63, wherein the sensor radiates beams onto theregistration marks and the image density marks, and receives theradiated beams which are reflected from the registration marks and theimage density marks to detect the registration information and the imagedensity information based upon the reflected beams.
 65. An apparatuscomprising: a belt moving in a first direction perpendicular to a seconddirection; a plurality of marks on the belt, each of the markscomprising a component in the second direction; and a sensor tosimultaneously detect an offset of the marks in the first and seconddirections.
 66. The apparatus of claim 65, wherein the sensorsimultaneously detects a skew and a printing width error of the marks.